1. Field of the Invention
The present invention generally relates to the field of plasma switches, and more specifically to a plasma switch combining a hollow, thermionically heated cathode with a control grid structure to enable switching (closing and opening) of high voltages and currents with a low voltage drop across the switch.
2. Description of the Related Art
Solid state switching devices including Gate-Turn-Off (GTO) thyristers and Integrated-Gate-Bipolar-Transistors (IGBT) are capable of fast-switching, low voltage drop, and cryogenic operation. Such devices have been heretofore used in high-power inverter/converter systems that convert high power from a low DC voltage to a high DC voltage. However, solid-state switches are low-voltage (&lt;1 kV) devices, which either force the prime source to operate with a low voltage output or require the switch array to operate with many thyristers or IGBT's connected in series. Low-voltage operation requires high circulating current and causes high joule-heating losses in an associated transformer. Moreover, transformer coupling is poor because of the necessity for a high primary-to-secondary turns ratio when a high voltage output is required. Series operation of the IGBT's multiplies switch-power loss and requires additional snubbing circuits to force voltage sharing and to arrest voltage transients during off-switching. These transients are particularly severe when using high-turns-ratio transformers that have significant self-inductance. The snubbing circuits add mass to the inverter and consume additional power. Solid-state devices are also not sufficiently rugged or fault tolerant since they can fail catastrophically when subjected to a single over-current or over-voltage event. In addition, solid-state devices cannot operate in high-temperature and high-radiation environments, such as in space, in connection with a nuclear reactor power source, or in ground installations or mobile systems where high temperatures are encountered near a diesel or turbine motor. A discussion of these problems in inverter/converter systems of the type to which the present invention applies is found in a textbook entitled "MODERN DC-TO-DC SWITCHMODE POWER CONVERTER CIRCUITS", by R. P. Severns et al, Van Nostrand Reinhold Co, New York 1985, pp. 95 to 97.
A low pressure plasma opening switch has recently been developed which overcomes the major drawbacks of the conventional solid state switching elements discussed above. The switch is referred to as the CROSSATRON Modulator Switch (CROSSATRON is a trademark of the Hughes Aircraft Company, the assignee of the present invention). Details of this switch are provided in U.S. Pat. No. 4,596,945, issued June 24, 1986, to R. W. Schumacher et al, which is assigned to the Hughes Aircraft Company, and in a text edited by Guenther et al., Opening Switches, chapter entitled "Low-Pressure Plasma Opening Switches", Schumacher et al., pp. 93-129, Plenum Publishing Corp, 1987.
The CROSSATRON switch is a secondary-electron-emitter, cold cathode device which employs a controlled diffuse discharge to both close and open pulsed-power circuits at high speed and high repetition frequency, and enables operation at substantially increased voltage and current levels compared to the solid state switching devices. In addition, the CROSSATRON switch is rugged and fault-tolerant, and can be cooled cryogenically. However, the switch typically produces a forward voltage drop on the order of 500 V, making it unsuitable for certain applications.
Whereas the CROSSATRON switch utilizes cold cathode, crossed-field discharge for plasma formation, a hollow cathode plasma source such as disclosed in U.S. Pat. No. 4,800,281, issued Jan. 24, 1989, entitled "COMPACT PENNING-DISCHARGE PLASMA SOURCE", to W. S. Williamson, uses a thermionic cathode to form a plasma. A hollow cathode configuration eliminates the necessity of a magnetic field for plasma confinement, which is necessary to attain a sufficiently high level of back ion bombardment in a cold-cathode configuration, although such may optionally be provided as discussed in the patent. In a hollow cathode discharge, an electric potential, and optionally external heat, are applied to raise the cathode to thermionic discharge temperature and initiate a glow discharge. The hollow configuration of the cathode and the electron-reflecting cathode sheath formed thereon functions to increase the lifetime and path length of thermionically emitted electrons inside the cathode. This greatly increases the probability of multiple ionizing collisions of the electrons with gas molecules in the hollow cathode. This enables the plasma discharge to be sufficiently contained for purposes of sustaining the discharge without the need of any other external means. A characteristic of a hollow cathode plasma source is the very low forward voltage drop, which can be on the order of approximately 10 volts (when xenon gas is used), across the cathode and anode of the source.
Another example of a hollow cathode gas discharge device used as a source of electrons or ions is found in U.S. Pat. No. 3,831,052, entitled "HOLLOW CATHODE GAS DISCHARGE DEVICE", issued Aug. 20, 1974, to R. Knechtli. A preferred application of the device is as an electron source for a gas laser with high energy ionization.